Kerry Back
\[r_{\text{new}} = r_f + (EV/E) (r_{\text{old}}-r_f)\]
\[\frac{E}{EV} \left[r_f + \frac{EV}{E}(r_{\text{old}}-r_f)\right] + \left(1-\frac{E}{EV}\right)r_f\]
\[EV - tD = E + D - tD = E + (1-t)D\]
\[\frac{E+(1-t)D}{E}= 1 + (1-t)\frac{D}{E}\]